This invention relates to ignition timing control of an engine in which a knocking state of the engine is detected and a supercharged pressure is feedback-controlled, amd more particularly to an apparatus for controlling the ignition timing to avoid the occurrence of knocking by decreasing the supercharged pressure and holding an intake air pressure indicating signal at just before the occurrence of knocking in order to compute the subsequent ignition timing in accordance with the held signal.
In an internal combustion engine having a supercharger, knocking may easily happen as the supercharged pressure is increased. Therefore, an apparatus for detecting knocking and obviating it is desirable. In currently known apparatuses for this, either the ignition timing is delayed or the supercharged pressure is decreased. Although the former method has a danger of causing such problems as a temperature rise of the exhausted gas and deterioration of the fuel combustion rate, the latter method has such merits as causing a decrease in the temperature of the exhausted gas and causing no degrading in the combustion rate.
Typical signals which represent the operation state of the engine for controlling the ignition timing are those of engine speed (revolution number of engine) and the intake air pressure. If knocking is to be obviated by delaying the ignition timing, a basic ignition timing shuld be set substantially in a trace knock region by referring to the engine speed and intake air pressure. In order to obviate knocking, the actual ignition timing is further delayed from the basic ignition timing. Like this the parameters of engine speed and intake air pressure used for the determination of the basic ignition timing are not changed with the obviation of knocking. On the other hand, if the knocking is to be obviated by decreasing the supercharged pressure, the intake air pressure for determining the basic ignition timing varies. Thus, if a controller is constituted in such a manner that the ignition timing be uniquely determined by the engine speed and the intake air pressure, it will meet following troubles. Namely, on the assumption that the ignition timing is set in a trace knock region, when the supercharged pressure or the intake air pressure is decreased, it may happen with a large probability that the ignition timing advances to a new trace back region corresponding to the decreased intake air pressure. This would cause further knocking and hence the intake air pressure must be again decreased. With repetition of such a cycle, it would become impossible to supply any supercharged pressure to the engine. On the other hand, if the ignition timing is set with a too large margin against knocking, the apparatus for avoiding knocking itself would lose its meaning.